1. Field of the Invention
The present invention pertains to methods and systems for the production of liquefied petroleum gases (LPG). Specifically, it pertains to systems and methods for converting natural wellhead gas to LPG.
2. Brief Description of the Prior Art
Gas wells of sufficient volume are normally connected to a gas pipeline for transporting the natural gas to natural gas markets. While the gas may be separated from hydrocarbon liquids exiting the well and dried, it is basically left in its natural form for such transportation to market.
Some natural gases may be converted to liquefied petroleum gas (LPG). The primary advantage of LPG is its ability to be transported by truck or rail to ultimate points of use not capable of supporting a pipeline. For example, LPG is widely used on farms for heating, crop drying and dehydration, tobacco drying, flame cultivation, irrigation pumps, cotton ginning, and stationary engine operations. In addition, many farmers use LPG to operate their tractors. Many buses, trucks, fork-lifts and the like use LPG.
Large industrial and manufacturing companies have found LPG to be an answer to heating problems because of its high purity, constant quality and competitive cost as compared with other types of gases and liquid fuel. Many small city gas plants utilize LPG. Refineries may use LPG in the manufacture of various grades of gasoline and high-octane motor fuels. LPG is used as basic raw material in the manufacture of many plastic synthetic fibers, synthetic rubber, etc.
In recent years, gas wells which were isolated or otherwise not easily connected to a gas pipeline have been provided with processing units for converting some of the natural gas to LPG. Thus, the LPG produced thereby can be stored in a tank for transportation by truck or railcar. However, such processing units usually require gas production in excess of 1000 MCFD. This eliminates many wells which produce less than 1000 MCFD. Thus, such wells must be either shut in or, if produced for their liquid hydrocarbon content, must be flared, wasting the gas produced thereby.
In U.S. Pat. No. 4,419,114, which issued from the parent of the present application, a system and method are disclosed for converting natural wellhead gas to LPG. The term "natural wellhead gas" as used herein refers also to "casing head" or "residue" gas. The method and system of U.S. Pat. No. 4,419,114 are specifically designed for wells producing less than 1000 MCFD. Thus, previously uneconomical wells may be produced so as to contribute to solving the current energy shortage while producing significant income to the well owner and the gas processor.
The system of U.S. Pat. No. 4,419,114 comprises a gas compressor connected to the wellhead for compressing natural gases received therefrom, refrigeration means downstream of the gas compressor for cooling the compressed gases therefrom and produce separator means downstream of the refrigeration means for receiving cooled and compressed gases discharged from the refrigeration means and separating LPG therein from gases remaining in vapor form. A storage tank may be provided for receiving LPG exiting from the product separator means for storing the LPG under pressure therein. The system of U.S. Pat. No. 4,419,114 is especially characterized by providing means for unloading the gas compressor in the event of its suction pressure falling below a predetermined level. Thus, the system is continuously operable even when incoming pressures fall below a safe level, e.g. 2 psig. The gas compressor simply cycles or idles under such conditions, preventing build-up of temperatures or entry of air into the system which might well occur without such an unloading means and if such did occur could cause hazardous explosions. While the system of U.S. Pat. No. 4,419,114 has proven to be economical and practical, the means for unloading the gas compressor is not without problems.